Experimental and numerical analysis of B1(+) field and SAR with a new transmit array design for 7T breast MRI

Abstract

Developing a radiofrequency (RF) coil system that produces a uniform B1(+) field (circularly polarized component of the transverse magnetic field responsible for excitation) and low specific absorption rate (SAR) is critical for high performance ultrahigh field human imaging. In this study, we provide the design of a new eight channel radiofrequency (RF) transmit (Tx) array for breast MRI at 7T. A numerical analysis utilizing an in-house finite difference time domain (FDTD) package was carried out in (1) four breast models, (2) homogeneous spherical model and (3) full body model to calculate the B1(+) intensity (μT) and homogeneity represented by coefficient of variation (CoV=standard deviation/mean) in the proposed RF array design. The numerical results were compared with that measured in breast phantom (Bphantom) and homogeneous spherical phantom at 7T MRI and showed very good agreement. Average and peak SARs were also calculated in the four breast models and the temperature rises due to the operation of the RF array were also measured in the Bphantom. The proposed RF array; which can operate in a single or multi transmit modes, demonstrates homogeneous RF field excitation with acceptable local/average SAR levels for breast MRI at 7T.

Keywords: 7T; B(1)(+); Breast; FDTD; RF coil; SAR; Tx/Rx array.

Figure 1

Constructed TTT Tx array, schematic diagram of one side of the array and breast phantom (AD). (A) One side of TTT RF array showing four excitation ports. Phases (90° increment) applied in the side #1 were in a clockwise rotation whereas the same phases were applied in counterclockwise rotation in side #2. Red arrows represent four excitation ports that can be matched by pulling/pushing the solid copper rods (red bar represent matching rods and yellow bar represent tuning rods). (B) Schematic of one side of Tx coil showing electrical connection between each ports and RF shield. Red/yellow bars represent the inner matching and tuning copper rods connected to the RF ground and orange box represents RF shield. Black lines and bars represent the electrical connections to the four excitation center pins. (C) Top view of assembled RF coil. RF shield box (green dotted line) was attached to the two sides. (D) Picture and MR images of the BPhanom in three different planes. Dimensions of the BPhantom are 120*100*90 mm³ and the volume is 500 cc.

Figure 2

FDTD model of TTT Tx array with 3D breast model co-centered at the bottom of the coil (A and B). Total grid of 162 * 162 * 168 cells was used in the computational domain. Sagittal (A) and transverse (B) plane views of the RF coil and the breast model are shown. Two sides of 170 × 170 mm² RF array were 127 mm apart and RF shield box dimensions were 127 × 170 mm². (C) Eight excitation ports (no RF shield) are shown. Red dot represents the excitation source points at each port

Figure 3

Permittivity map of four 3D anatomically detailed breast models used in FDTD calculations. (A) mFBM: <25% glandular (B) sFGBM: 25–50% glandular (C) hDBM: 50–75% glandular and (D) vDBM: >75% glandular. The phantom models consisted of six tissues with air surrounding. The colorbar represents the relative permittivity of the tissues in the model.

Figure 4

S-matrix calculated from four breast models and spherical model and measured from the Bphantom and spherical phantom. (A) FDTD calculated S11 and S13 comparison between four different breast models and the spherical model. (B) Measured S11 and S13 of the spherical phantom utilizing vector network analyzer (showing very good agreement with (A)). Note that, the high coupling (~−3 dB) at S13 is intrinsic characteristic of the TTT RF coil. (C) The mean transmission/reflection coefficient comparison between the simulations and experiments. The simulations were obtained with the four breast models and the spherical model whereas the experiments were obtained with the spherical phantom and the Bphantom.

Figure 5

(A) Simulated/measured B₁⁺ distribution from the homogeneous spherical phantom at three different planes (B₁⁺ was normalized with the local maximum B₁⁺ intensity). (B) Signal profile comparison (white dotted line in (A)) in the coronal and transverse planes; x axis represents pixel points. Diameter of the phantom/model is 100 mm.

Figure 6

(A) Mean B₁⁺ field intensity (μT) per 1W of RF power supplied by the system calculated in the four breast models (simulations) and measured experimentally in the Bphantom. Note, white dotted line is the border of the chest wall used for CoV calculation labeled “without the chest wall”. B₁⁺ map was normalized to the maximum B₁⁺ measured in the model (2.38 μT in mFBM). (B) Mean CoV at three different planes. (C) CoV over the entire volume.

Figure 7

(A) SAR (W/kg) for a mean B₁⁺ field of 2μT calculated in the four breast models. Peak SAR was measured close to the skin and chest wall region in all four models. The locations of the four thermal fiber optic probes used for the temperature measurements are shown as red and green bars. Three probes were inserted at the periphery of the phantom (30 mm depth, red bars) and one probe was placed at the center of the phantom (50mm depth). Note, SAR map in four models were normalized to the highest calculated peak SAR (1.75 W/kg in sFGBM). (B) Temperature measurements at four locations in the Bphantom. (C) Zoomed-in view of the temperature rises measured from Ch1 and Ch4 during first RF heating. Note RF was applied at 600s and each scan was 2 min long and sequentially repeated for 5 times.